There are numerous applications that can use the location of a remote unit in a wireless communication system. For example, the location of the remote unit can be used for emergency services, so that police, fire and/or medical services can be dispatched to the location of the subscriber requesting such an emergency service. In addition, the location of the remote unit can be used in detecting fraudulent use of the communication system, police investigations, and the like. Furthermore, the location of a remote unit can be used to improve performance of the wireless communication system. For example, wireless communication systems that utilize beam-forming transceivers can tune the beams based at least in part on the direction of arrival (DOA) of a signal transmitted from the remote unit. DOA is often an important parameter used in location determination.
Wireless communication systems currently have the capability for determining the location of a remote unit. However, increased resolution for the DOA of a signal from the remote unit is continually sought for these communication systems. (See H. Krim and M. Viberg, “Two Decades of Signal Processing Research,” IEEE Signal Processing Magazine, Vol. 13, No. 4, Jul. 1996 (hereinafter referred to as the “Two Decades Reference”), which is hereby incorporated by reference.) In order to improve the resolution for the DOA of a signal from a remote unit in a wireless communication system that utilizes an array of antennas (i.e., antenna array), the methods and apparatus for determining the DOA preferably address array distortions.
Array distortions, which collectively refers to a wide variety of distortions and modeling errors, can be attributed to any number of factors, including physical manufacturing differences between antenna arrays, mutual coupling between elements of antenna arrays, mutual coupling between elements of arrays and metallic resonators, differences in the connection lengths between array elements, characteristic differences of transceivers that are processing signals received from the elements of antenna arrays, and so forth. These array distortions are known to be sources of degradation to DOA estimation accuracy, and methods and apparatus have been proposed to compensate for array distortions.
For example, direction-finding (DF) techniques have been developed that determine DOAs of multiple simultaneously detected co-channel signal sources, including the Multiple SIgnal Classification (MUSIC) and the Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT). (See R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Transactions on Antennas and Propagation, Vol. AP-34, No. 3, March 1986, pp. 276-280, which is hereby incorporated by reference, and U.S. Pat. No. 4,750,147 titled “Method for Estimating Signal Source Locations and Signal Parameters using an Array of Signal Sensor Pairs,” issued to Richard H. Roy, III et al. on Jun. 7, 1988, which is hereby incorporated by reference.) MUSIC utilizes pre-recorded measurements of the array manifold, so this technique is largely immune to array distortion effects. However, MUSIC is an unstructured method and therefore computationally intensive as each DOA estimate requires a full scan of the entire array manifold. ESPRIT is less computationally intensive as compared to MUSIC as it does not utilize a-priori measurements of the array manifold. However, ESPRIT generally requires identical pairs of sensors, which makes it susceptible to many mechanisms that induce array manifold errors.
Calibration of the array manifold, which is commonly used to specify the response of the antenna array to a stimulus and an idealized response and a composite of the idealized response with the effect of array distortions, continues to be a subject that is investigated for increasing the accuracy of DOA estimates. For example, investigations have been conducted to identify the array manifold distortions that are present in complex environments. (See A. Leshem and M. Wax, “Array Calibration in the Presence of Multipath,” IEEE Transactions on Signal Processing, Vol. 48, No. 1, January 2000, pp. 53-59(hereafter referred to as the “Leshem Reference”), which is hereby incorporated by reference.) These investigations have shown that calibration for array manifold distortions in complex environments does provide the desired increases in the accuracy of DOA estimation.
In view of the foregoing, is should be appreciated that it would be desirable to provide methods and apparatus for determining a direction of arrival (DOA) of a signal. It should also be appreciated that it is desirable to provide such methods that are resistant to array manifold distortions and that are computationally efficient. Furthermore, additional desirable features will become apparent to one skilled in the art from the drawings, foregoing background of the invention and following detailed description of a preferred exemplary embodiment, and appended claims.